Known image-forming apparatuses are divided into various types, including electrophotographic apparatuses, heat-transfer apparatuses, and inkjet apparatuses. Among them, electrophotographic apparatuses are superior to other types of image-forming apparatuses in terms of high speed, high image quality, and quiet operation, and thus have increasingly become widespread in recent years. Electrophotographic apparatuses are further divided into various types, including multiple transfer apparatuses and apparatuses having an intermediate transfer member, which are well known types. Other types include multiple development apparatuses, which form color images on a photosensitive member and simultaneously transfer the images to create a single image, and in-line apparatuses, which have image-forming units (process stations) of different colors arranged in line to transfer developed images to a transfer material fed by a transfer belt. Among them, in-line apparatuses are advantageous in terms of high speed, high image quality, and a small number of times of image transfer.
FIG. 3 illustrates the structure of an in-line image-forming apparatus. In FIG. 3, an electrostatic-attraction feed belt (hereinafter referred to as an ETB) 1 is suspended on a drive roller 6, a roller 7 disposed opposite an attraction roller 5, and tension rollers 8 and 9. The ETB 1 rotates in a direction indicated by the arrow. Process stations 201 (yellow), 202 (magenta), 203 (cyan), and 204 (black) are arranged in line along the circumferential surface of the ETB 1, each including a photosensitive member disposed in contact with one of transfer rollers 3 with the ETB 1 disposed therebetween. The attraction roller 5 is disposed in contact with the roller 7 upstream of the process stations 201, 202, 203, and 204. A transfer material P is biased when passing through a nip between the attraction roller 5 and the roller 7. A power supply 16 charges the ETB 1 through the roller 5. The transfer material P is electrostatically attracted to the ETB 1 and is fed in the direction indicated by the arrow.
An example of the ETB 1 used in the known art is a resin film having a thickness of about 50 to 200 μm and a volume resistivity of about 109 to 1016 Ωcm and formed of, for example, polyvinylidene fluoride (PVdF), an ethylene-tetrafluoroethylene copolymer (ETFE), a polyimide, polyethylene terephthalate (PET), or a polycarbonate. Another example is a resin film including, for example, a base layer having a thickness of about 0.5 to 2 mm and formed of a rubber, such as an ethylene-propylene-diene monomer (EPDM), and a surface layer formed by dispersing a fluoropolymer, such as polytetrafluoroethylene (PTFE), in a urethane rubber. Such resin films may have, for example, an acrylic coating to increase glossiness.
The image-forming process of the in-line image-forming apparatus will now be described. First, the image-forming process of the process stations 201, 202, 203, and 204 is described below. The description focuses on the image-forming process of the yellow process station 201, although the other process stations 202, 203, and 204 have similar image-forming processes.
FIG. 4 illustrates the structure of the process station 201. A charging unit 212 uniformly charges a photosensitive member 211. An optical exposure system 213 scans the photosensitive member 211 with light 214 to form a latent image thereon. A developing roller 215 develops the latent image to form a toner image on the photosensitive member 211 by depositing toner stored in a toner container 216. Residual toner left after a transfer process described below is scraped off by a cleaning blade 217 and is recovered into a waste toner container 218.
Next, the transfer process is described below.
In reversal development, a commonly used development process, a negatively charged toner, for example, is used for development of exposed areas on a negatively charged organic photoconductor (OPC). Hence, bias power supplies 4 apply a positive transfer bias to the transfer rollers 3. The transfer rollers 3 used are typically low-resistance rollers.
In actual printing, the image-forming process, the transfer process, and the feeding of the transfer material P are performed at such timings that toner images of the individual colors are aligned on the transfer material P. These processes are controlled according to the moving speed of the ETB 1 and the distances between the transfer positions of the process stations 201, 202, 203, and 204. A single toner image is created on the transfer material P each time the transfer material P passes through the process stations 201, 202, 203, and 204. After the creation of the toner image, the transfer material P is allowed to pass through a known fusing unit which in turn fuses the toner image on the transfer material P.
Image density varies with the temperature/humidity conditions under which the image-forming apparatus is used and the frequency with which the process stations 201, 202, 203, and 204 are used. Such density variations are corrected by image density control described below. FIG. 13 is a block diagram of a control system of the image-forming apparatus.
In FIG. 13, an image-forming apparatus 30 includes a controller 31 and an engine 32 including a control unit 33. A host computer 20 or the controller 31 directs the image-forming apparatus 30 to form toner patch images of the individual colors for density control on a photosensitive member, an intermediate transfer belt (hereinafter referred to as an ITB), or an ETB. A density sensor 13 detects the toner patches. The control unit 33 includes a memory 35 for storing the detection results and a CPU 34 for calculating the densities of the toner patches. The control unit 33 feeds back the calculation results to process conditions, such as high-voltage conditions 37 for charging and development and the power of a laser 36, to adjust the maximum density for each color and cancel out γ characteristics (nonlinear input/output characteristics specific to electrophotography) for each color, thus controlling halftone characteristics. In general, the density sensor 13 illuminates the toner patches with light from a light source and detects the intensity of reflected light with a light-receiving element. The intensity signals of the reflected light are subjected to A/D conversion, are processed by the CPU 34, and are fed back to the process conditions.
The image density control is intended to maintain a constant maximum density for each color (hereinafter referred to as Dmax control) and to maintain halftone characteristics linear with respect to image signals (hereinafter referred to as Dhalf control). The Dhalf control also has great significance in maintaining a constant color balance and preventing spatters from toner characters of overlapping colors and defective fusion due to excessive amounts of toner deposited.
In the Dmax control, generally, image-forming conditions are controlled by detecting toner patches formed under different image-forming conditions using an optical sensor and determining such conditions that the desired maximum density can be achieved according to the detection results. Halftone toner patches are often formed in the Dmax control. The detection of solid images causes difficulty in providing sufficient detection accuracy because of the narrow range of variations in sensor output relative to variations in the amount of toner. The Dmax control based on the detection of toner patches is not required for systems in which the maximum density for each color depends more largely on the thickness of an OPC and image-forming environments than on image-forming conditions. Such systems are advantageous in terms of usability and toner consumption because the image-forming conditions of the systems can be determined according to environment detection results and CRG tag information.
The Dhalf control cancels out γ characteristics to prevent formation of unnatural images due to deviations of output densities from input image signals and thus enable image processing with linear input/output characteristics. In the Dhalf control, toner patches corresponding to different input image signals are detected using an optical sensor to determine the relationship between input image signals and densities. The controller 31 then converts image signals input from the host computer 20 on the basis of the signal-density relationship to determine the density desired for the input image signals. The Dhalf control is generally performed after the image-forming conditions are determined by the Dmax control.
The toner patches formed on the ETB 1 are electrostatically recovered into the process stations 201, 202, 203, and 204 by a cleaning process. The toner of the toner patches, as well as the toner left after the transfer process, is attracted to the photosensitive members 211 by applying a reverse bias thereto and is scraped off by the cleaning blades 217. The method used for cleaning the ETB 1 is not limited to the electrostatic recovery described above and can also be performed by physical recovery, for example, by bringing a cleaning blade into contact with the ETB 1.
If a density sensor is disposed opposite one of rollers supporting a belt-like rotating medium, foreign matter adhering to the opposite roller, for example, can produce an adverse effect on sensor output despite only small positional variations in glossiness existing on the belt-like rotating medium. This problem is particularly serious for detection of toner patches with lower densities. This problem remains the same for a system with large positional variations in glossiness on a belt-like rotating medium even if sensor outputs measured on toner patches are normalized with respect to those measured on the rotating medium to cancel out variations in the amount of reflected light due to the rotating medium. This problem can be avoided in the normalization method if the circumference of the rotating medium is equal to an integral multiple of that of the opposite roller.
In practice, however, such an integral multiple relationship is often difficult to select because the selection of the circumferences of the rotating medium and the opposite roller is restricted by, for example, the height of the image-forming apparatus. Variations in the amount of light reflected by the rotating medium can be correctly estimated by a modified averaging process if the variations form a sinusoidal curve due to variations in the outer diameter of the opposite roller.
In an example of the averaging process, the amounts of reflected light are detected on the rotating medium or toner patches at regular intervals over a range equivalent to the circumference of the opposite roller. The detection results may be averaged to cancel out misdetection due to the opposite roller. The amount of reflected light can then be correctly estimated by subtracting variations in the amount of reflected light due to the rotating medium. Variations in the amount of reflected light due to foreign matter adhering to the opposite roller, however, are difficult to cancel out by the averaging process because such variations form a periodic but asymmetrical wave.